It's well known that in several instruments commonly employed in ophthalmology it is very important that a patient holds his/her head in a predetermined position, that is symmetrical for the right and for the left eye.
This need is particularly felt in the instruments for measuring the vision threshold, such as optical field meters and perimeters.
A perimeter is an instrument consisting of a hemispherical dome onto which light spots are projected of intensities and sizes determined at will by the examining operator.
A patient, sitting in front of the dome, lays his/her head on an adjustable device, commonly referred to as chin-rest, and fixes his/her selected eye, while the other is blindfolded, on a spot at the center of the dome. The patient must press a button as soon as he/she sees the luminous stimulus projected onto the dome.
The interpolation of all the peripheral spots seen gives a map that defines the visual field.
The operator must be sure that the patient doesn't lose his/her staring during the test. Such a check is performed by means of a telescope or of a telecamera arranged behind a hole at the center of the hemispherical dome.
In the more sophisticated instruments an optoelectronic system is provided, which automatically senses the losses of staring, following which the projection being performed is unvalidated or momentarily suspended.
Obviously, these systems, being based on the detection of the movements of the eye's image taken by the telescope or by the telecamera, are more sensitive to the lateral movements of the head than to the rotation of the eye with respect to the point fixed with the eye.
That makes it even more important that the chin-rest keeps the patient's head still as much as possible.
To this end, a belt which arrests the head has been adopted in some chin-rests.
Such a solution is increasingly discarded as it causes a considerable feeling of constraint to the patient.
Moreover, in the apparatuses used at the present day, the point to be fixed with the eye coincides with the hole at the center of the hemispherical dome, so the visual range cannot be examined correspondingly to this hole as it is not possible to project luminous spots there.
In addition, by constantly staring at a black hole on a white surface, an effect is created by which a retina area much greater than that of the hole itself is sensitized, whereby the test turns out to be not very reliable correspondingly to this area.
That involves, in addition, a disagreeable effect, whereby the patient's difficulty to keep his/her head firm for a long time increases.